The present invention relates to apparatus for sensing rotational speed and, more particularly, to speed sensors for sensing the speed of rotation of a fan or fan-like structure about an axis of rotation.
Various electrical devices are known for sensing rotational speed, including the speed of rotation of a multi-bladed fan. For example, electric sensors using magnetic fields or capacitive effects are known in which the rotating structure generates an electrical pulse signal as a function of the rotational speed. For example and in the context of a rotating fan, magnetic sensors are known in which a magnetic pick-up coil or a Hall effect sensor is positioned in close proximity to the ends of the fan blades. As the fan rotates, the ends of the blades move pass the sensor to perturb the magnetic field adjacent the sensor to generate a sequence of electrical pulses having a repetition rate that is proportional to rotational speed. Capacitive-type sensors operate in a similar manner and usually employ a simple bridge circuit that detects a change in the capacitance between the sensor and the blade end to produce a pulse output. Other types of sensors use a light beam that is periodically interrupted, e.g., chopped, by the motion of the fan blades to cause a photo-sensor to produce a pulse output having a pulse repetition rate that varies with the rotational speed of the fan blade.
In general, the type of speed sensor systems discussed above perform there intended function, although sensors that can be characterized as electrical tend to produce EMI and, conversely, are EMI sensitive. In certain environments, for example, in sensing the speed of rotation in aircraft fan-jets or other turbo-machinery, shielding is required to prevent or minimize adverse EMI effects. In general, shielding adds to the capacitive impedance in any electrical system and can adversely limit the upper operational frequency of the system as well as add undesirable weight in an airborne application. Traditional photo-electric systems, in addition to requiring shielded components in many applications, are sensitive to and can be operationally compromised by the accumulation of debris and other light obscuring contaminants on their optically active surfaces or in the optical path between the light source and its receiver.
One of the trends in the instrumentation field is the incorporation of optical fibers into the instrument system to take advantage of their lightweight, immunity to electric and magnetic fields, and their high bandwidth. For example, the output of electrical sensors can be converted to light pulses and transmitted via unshielded optical fiber to a receiver for further processing. In the context of prior photo-electric systems, light energy can be also be directly launched into a fiber for transmission to a remote processing location. In this latter situation, the optical path is nonetheless `open` and sensitive to light obscuring contaminants in a manner analogous to the traditional photo-electric system. Ideally, any system that uses optical energy should be closed, that is, not subject to the effects of light obscuring contaminants.